Dispersion-strengthened zirconium products

ABSTRACT

A method of producing dispersion-strengthened zirconium products containing fine particles of yttria, magnesia, cerium oxide or beryllium oxide in a matrix of zirconium or a zirconium alloy. Preferred products are such containing dispersed fine particles of yttria. In the method hydrided zirconium or zirconium alloy in a manner known per se is in a pulverulent state mixed with a fine powder of the oxide, and the mixture is then heated in vacuum with a view to expel hydrogen. Simultaneously or subsequently, the mixture is - in a compressed condition - sintered. This heating operation for expelling hydrogen and sintering the mixture is carried out at a temperature below 800*C, and preferably below 750*C. Prefered dispersion-strengthened zirconium products are characterized by having the metal oxide particles, of an average size not substantially exceeding 0,5 Mu and preferably between 0,01 and 0,5 Mu , dispersed in the said matrix having an average particle size not exceeding 5 Mu and preferably between 1 and 3 Mu .

United States Patent [1 1 Adolph et al.

[ 1 Dec.4,1973

i 54 l DlSPERSlON-STRENGTHENED ZIRCONIUM PRODUCTS [75] Inventors: EivindAdolph; Niels Hansen; Jens Christian Bailing Jensen; John Kjoller, allof Roskilde, Denmark; Paul Donald Parsons; Edward David llindle; DavidJohn Marsh, all of Lancs, England [73] Assignee: Atomenergikommissionen,

Copenhagen, Denmark [22] Filed: Aug. 18, 1971 [21] Appl. No.: 172,616

[30] Foreign Application Priority Data Aug. 2], 1970 Denmark 4318/70[52] US. Cl 29/1815, 75/211, 75/206, 75/225 [51] Int. Cl. B22f 1/00,B22f 3/00, B22f 3/14 [58] Field of Search 75/21 1, 206, 225; 29/ 182.5

I 56] References Cited UNITED STATES PATENTS 3,507,630 4/l970 Rezek75/206 OTHER PUBLICATIONS Lustman et al., The Metallurgy of Zirconium,

McGraw-l-lill, 1955, pp. 291 TN 799 Z 5L 8 Primary ExaminerCarl D.Quarforth Assistant ExaminerB. Hunt Attorney-Beveridge & De Grandi [57]ABSTRACT A method of producing dispersion-strengthened zirconiumproducts containing fine particles of yttria, magnesia, cerium oxide orberyllium oxide in a matrix of zirconium or a zirconium alloy. Preferredproducts are such containing dispersed fine particles of yttria. In themethod hydrided zirconium or zirconium alloy in a manner known per se isin a pulverulent state mixed with a fine powder of the oxide, and themixture is then heated in vacuum with a view to expel hydrogen.Simultaneously or subsequently, the mixture is in a compressed conditionsintered. This heating operation for expelling hydrogen and sinteringthe mixture is carried out at a temperature below 800C, and preferablybelow 750C. Prefered dispersion-strengthened zirconium products arecharacterized by having the metal oxide particles, of an average sizenot substantially exceeding 0,5 y. and preferably between 0,0l and 0,5dispersed in the said matrix having an average particle size notexceeding Sp. and preferably between 1 and 3p" 18 Claims, 2 DrawingFigures DISPERSION-STRENGTHENED ZIRCONIUM PRODUCTS BACKGROUND OF THEINVENTION This invention relates to dispersion-strengthened zirconiumproducts and to a method of producing such products which possessincreased strength and at least as good a corrosion resistance as doprior art zirconium materials that have so far been employed in thereactor technique, in particular for pressure tubes and forencapsulating fuel elements.

The dispersion-strengthened zirconium products dealt with here containfine particles of yttrium oxide (Y O magnesium oxide, cerium oxide (CeO,or c8 0,) or beryllium oxide dispersed in a zirconium or zirconium alloymatrix. As such a zirconium alloy, zircaloy in particular is employedwhich contains small amounts of tin, iron and chromium as alloycomponents.

Prior art dispersion-strengthened zirconium products of this typecontaining fine Y O particles as the dispersant have been described byKC. Antony and H.H. Klepfer in J. Less-Common Metals, 1965), pages 36 to46, and by J. Rezek and 8.6. Childs in 1. Nuclear Materials," 26 (1968),pages 285-299. When producing these known materials, the startingmaterial is powdery. hydrided zirconium or zirconium alloy which, in anargon atmosphere, is very thoroughly mixed with yttrium oxide powderhaving a very small average particle size varying from 0.05 1. to aboutBy. and the quantity of added yttrium oxide powder can be such that, inthe dispersion-strengthened product ultimately obtained, it constitutes5-l0 percent by volume, for example. The mixture obtained is compressedand heated in a vacuum to a high temperature with a view to expellinghydrogen to a very low content of hydrogen of about l0-20 ppm or less,and for the purpose of sintering, which process takes several hours. Inthe course of this process of expelling hydrogen and of sintering,temperatures ranging from l,00OC to above l,l0OC have been used. Whileit was a known fact that lower temperatures might be beneficial for thestability of the finest Y O particles, temperatures in the said rangehave been regarded as necessary not only for sintering but also withrespect to the important effective expelling of hydrogen. The sinteredproducts obtained have been subjected to a subsequent mechanicalcompacting process by means of extrusion or hot rolling at temperaturesvarying from 800-950C. In the prior art cases dealt with here, anextrusion ratio of 10:1 or a reduction ratio of 50:1 was used.

Although the dispersion-strengthened zirconium products obtained in thisfashion are good, there still remains the need for an improvement in thestrength properties, and it has now proved that it is possible to obtainsuch an improvement, without any deterioration in the corrosionproperties, by the method according to the present invention.

DESCRIPTION OF THE INVENTION In the method of the invention, zirconiumor a zirconium alloy in a hydrided, powdery state is in a manner knownper se mixed thoroughly with powder of yttrium oxide, magnesium oxide,cerium oxide or beryllium oxide, having a very small average particlesize and the mixture is heated in vacuum for the purpose of expellinghydrogen and for simultaneous or subsequent sintering in a compressedcondition, said heating process for expelling hydrogen and for thepurpose of sintering being carried out at a temperature of below 800Cand preferably below aproximately 750C.

It is surprising that an effective expelling of hydrogen to values ofl0-20 ppm or less, together with simultaneous or subsequent sintering,resulting in improved strength properties, is possible when making useof such a lowering of the operating temperature. The sintering operationwill normally be carried out, at any rate so far as its final stage isconcerned, at a temperature in the region of 650 750C. As has beenmentioned, the heating operation is carried out in a vacuum, whereby thepressure may, by way of example, be kept between, or may vary between,10 and 10" mm Hg. At any rate during the later stage of the sinteringprocess it is best, in order to promote the expelling of hydrogen, tooperate at pressures of below about l0 mm Hg.

It is important that the particle size of the powder mixture be kept assmall as possible, and the metal oxide employed has preferably anaverageparticle size that does not substantially exceed 0.5 n and,preferably, is in the region of 0.01 to 0.5 .1. At the same time,

the particle size of the matrix hydride should not substantially exceed10p. and should preferably be in the region of 0.5-3 ;1..

Other embodiments and advantagesof the present invention will appearfrom the following, more detailed description of the inventioninconnection with theemployment of yttrium oxide. All the operations inconnection with the present method may be carried out in anyconventional equipment used in the manufacture of the said type ofdispersion-strengthened products.

The starting materials, hydrided zirconium or hydrided zirconium alloyand yttrium. oxide, are first milled to the desired particle sizementioned previously. They may be milled separately. and be subsequentlymixed, but it is more expedient that they be milled once they are mixedtogether. By way of example, the milling operation may be carried out ina suitable ball mill and should be effected in an inert atmosphere as,e.g., an atmosphere of pure argon.

If, for some reason or other, it is desired-to produce the hydridedzirconium or a hydrided zirconium alloy instead of using the commercialhydrided products, this may be done by heating the metal or alloy (forexample, in the shape of rods of a diameter of 4-6. mm.) in anatmosphere of pure hydrogen to a high temperature for several hours. Insuch process, hydrogen may conveniently be used at a pressure aboveatmospheric pressure, e.g., 1,000-1,250 mmv Hg. At such a hydrogenpressure it has been found to he appropriate to heat to a temperaturearound 900C. The heating operation may conveniently take place byincreasing the temperature at a rate of about 300C per hour and, whenthe desired temperature of around 900C has been reached, thistemperature is maintained for some hours, e.g., for approximately 4hours, whereupon cooling is effected. It has proved that the besthydriding results are achieved by carrying out the cooling operation intwo stages, in that, for instance after an appropriately slow coolingprocess to a temperature of about 400C, this temperature is maintainedfor some hours, conveniently 4-5 hours. The cooling to around 400C may,for instance, take place at a rate of approximately 50C per hour.Subsequent to said maintenance period of about 400C, the cooling toambient temperatur is proceeded with. The hydrided products so obtainedare friable and easily milled in a ball mill.

After the mixture of hydride and yttrium oxide has been milled, themixture is subjected to the hydrogen expelling and sintering treatment.To this end, the mixture may first be compressed into pellets, e.g., ofa diameter of 22 mm and a height of 30 mm, without any concurrentheating taking place, or, at any rate, without any substantial heating,and this compressing operation may conveniently take place at a pressurein the region of 100-130 Kp/mm The compressed body or bodies are thenplaced in a conventional sintering furnace, in which the sinteringprocess can be carried out in a vacuum. 1n carrying out this process ithas proved to be expedient to first maintain the temperature for somehours at a lower value of around 600 and up to about 700C and,thereupon, for some hours at a higher temperature. For example one mayproceed in the way that the sintering furnace is evacuated to a pressureof approximately l mm Hg, after which the tempertaure is raised toaround 660C in the course of 3-4 hours, following which the pressure andtemperature are kept constant for about 15 hours. Subsequently, thepressure is reduced to from 10 to 10 mm Hg and the temperature is raisedto approximately 700C. These conditions are then maintained for about 15hours, after which the furnace is allowed to cool to ambienttemperature.

In place of the said preliminary compression operation without anyheating, the powdery mixture as such may first be vacuum heated andsubsequently be sin tered under pressure. For this purpose, the powdercan be introduced into a press conventionally used for such sintering,following which it is evacuated to a pressure of around mm Hg just as inthe previous instance. Thereafter it can be heated, for example, to atemperature of approximately 620C, and, once enough hydrogen has beenexpelled so as to render possible the maintenance of a stable vacuum of10" 10 mm Hg, the temperature is raised to 650-750C, the pressure on thematerial in the press being increased to 10-50 kp/mm. These conditionsare maintained until the material has been completely sintered togetherwhich as previously, will take some hours.

Following the sintering operation, the obtained sinter-product willnormally be subjected to a mechanical compacting process by means ofextrusion while heated, just as were some of the prior art productsmentioned in the foregoing. In the embodiment that is utilized in thepresent invention, however, a somewhat lower temperature is employed,the compacting operation preferably being carried out at a temperaturebelow 800C, and preferably below 750C. In detail, the compactingoperation conveniently convenient be carried out by first inconventional manner encapsulating the sintered material completely incopper (using a copper tube having a wall thickness of, for example, 1.2mm), and then introducing it into an extrusion press. After heating toaround 750C, the material is extruded at a pressure of 70-100 kp/mm. Thematerial is extruded in the form of a rod, an extrusion ratio of :1being employed. The copper is thereupon removed, which may be done bymeans of dissolving it in nitric acid.

It is also possible to subject the sintered material to a hot rollingoperation at a relatively low temperature,

e.g., 350-500C, just as it is possible to undertake several other,conventional forms of treatment, care having to be taken that there isno, or only an insignificant, particle growth occuring in the material,so that the average particle size remains at the level dealt with in theforegoing. This will have to be ensured, for instance, when it isdesired to subject the sintered product, in particular the extruded(possibly hot-rolled) product, to a stress-relieving treatment (orannealing) by employing a suitable thermal treatment. For the extrudedproduct, a suitable form of such thermal treatment can consist in aheating operation in vacuum for some hours, for example, up to 10 hours,at around 600C.

The quantity of the added yttrium oxide powder may vary within widelimits. However, the quantity of the yttrium oxide will normallyconstitute not more than about 15 percent by volume of the finalproduct, while the lower limit is determined solely by whether it ispossible to obtain any effect by the quantity added. The quantity may besubstantially below 5 percent by volume, in certain cases it may be aslow as approximately 1 percent by volume.

As a zirconium alloy any zirconium alloy may be employed, but formaterials to be used in nuclear reactors, the aforesaid zircaloy-alloycan expediently be used, e.g., zircaloy-Z.

In the f oficwving tab le l, dispersion stre n g thened zirconium alloysobtained by the method according to the invention are compared with theprior art, dispersionstrengthened zirconium alloys mentioned in theforegoing. Comparison is made both of products in which the matrixmaterial is zircaloy-2 and products, in which the matrix material iszirconium. Zircaloy-Z is designated TABLE I Strenth Properties at 500CZircaloy-2 and Zirconium matrix Flow Stress, 0.2% Offset kp/mm (10" psi)Ultimate Tensile Strength kp/mm (10 psi) Product 1. Zr-2 +10% Y,O; 26.9(38.3) 31.2 (44.4) 2. Zr-2 5% Y O 25.4 (36.0) 37.0 (52.5) 3. Zr-2 +10%Y,0, 31.6 (45.0) 40.8 (58.0) 4. Zr +10% Y,O,*) 21.8 (31.0) 24.1 (34.2)5. Zr 5% no, 17.6 (25.0) 25.7 (36.5) 6. Zr 10% 0 23.9 (34.0) 31.7 (45.0)7. Zr (sintered) 14.3 (20.3) 17.5 (24.9 powder) 8. Zr (sintered 9.0(12,8) 14.5 (20.6) powder) Tested after sintering.

By comparing the prior art product 1 with product No. 3 producedaccording to the present method, it will be seen that the flow stressand the ultimate tensile strength of the product according to theinvention are substantially higher. If the prior art product No. 4 iscompared with product No. 6, it will be seen that a better ultimatetensile strength is obtained in the product according to the invention.The strength of No. 4, as

indicated, is tested subsequent to sintering, while product No. 6 istested following extrusion. This could apparently explain part of thedifference in strength. It is seen, however, that the data stated forproduct No. 7

6 Furthermore, in Table II as detailed below, the corrosion propertiesof Zr-2 yttrium oxide (dispersionstrengthened product according to theinvention) have been stated and compared with some commercial zir-(produced in the same laboratory as product No. 4) for 5 caloy-2products. it appears that the corrosion resispure zirconium is muchhigher than the corresponding tance of the products according to theinvention is at data for product No. 8, produced by the applicants.least as high as is the case with the prior art products. The effectivedispersion-strengthening which is TABLE II achieved by the methodaccording to the invention, is, 2 therefore, much greater than the oneachieved by pro- Increase weight mg/dm 2 dlfferent alloys ducing productNo. 4. Thus, the difference in ultimate Posed to water and Steam at 400C and to pressure of tensile strength between product No. 4 and productP 100 atmospheres 2 Material increase in weight increase In weight No. 7is equal to 6.6 kp/mm while the difference in 111- ft after timatetensile strength between product No. 6 and z 2 d i y pp 14 y Mu 2 rto 5y product No 8 kp/mm Commercial Product l8 black 67 grey Moreover, amicroscopic examination shows that the Zr-2 od. Powde 38 H k t 0 priorart products Nos. 1 and 4 have a matrix, the partimgfil f :2 g 'gi clesizes of which vary from 5 to 10p, while the prod- Zr-Z 5 v/ l6 bright45 black with ucts Nos. 2. 3. 5 and 6 according to the invention have2:2: Tube it a matrix. the particle sizes of which varyjro m l toj comm]19 black EXAMPLES Composition of rods ZT+5V/0Yz03 Zr+10v/oY2O3Zr2+5v/oY2O Zr-2+10v/0Y:Oi

Weight of hydride, g 107. 88 102. 107. 88 102. 30 Weight of Y203, g 4.118. 21 4.11 8.21 Milling time, min 2X30 2X30 2X60 2X60 Cold compressionpressure, kp./

mm! 127 127 127 127 Sintering temperature, C.. 700 700 700 700 sinteringtime, hrs 15 15 15 15 Sintoring pressure, mm. Hg. 3X10-5 2X10-5 1X10-52X10-5 Extrusion temperature, C 750 750 750 750 Max. pressure, kp./mm.78 84 102 91 Min. pressure, kp./mm. 55 72 58 56 Ratio 15:1 15:1 15:115:1 Flow stress, kpJmmJ:

Ambient temp 53 71. 8 83. 6 90 350 59 47. 2 a7. 0 4s. r. 500 17. 7 26.326. 5 a4. 3 Ultimate tensile strength kp. mmfl:

Ambient temp 74. 3 83.0 99. 5 97. ii 350 39. 1 60.3 51. 5 5s. 2 500 25.3 36.3 36.8 41. 5 Elongation, percent:

Ambient temp 10. 7 2. 3 2. 7 1. 3 350 16 9. o 15. 2 1o 23. 5 25. 7 1s. a10 it is also shown that the yttrium oxide phase of the prior artproducts consists of comparatively large particles. As regards productNo. 4, they were measured to vary from 0.05 to 0.6 ,u., the average(linear) size being 0.2 ,1. For the sake of comparison, the particlesize of the yttrium oxide phase of products Nos. 2, 3,5 1 and 6 wasmeasured as varying from less than 0.01 to 0.5 1.1., with an average(linear) size of 0.04 p..

The products according to the invention create an impression of beingfully dense. They have good creep properties. Their strength propertiesare superior to those of the prior art products at temperatures of up to600C and, presumably, still higher temperatures.

in the accompanying drawings;

FIG. 1 shows the variation in ultimate tensile strength as function ofthe temperature, for the products Nos. 2, 3, 5 and 6 stated in Table l,as well as for a Zr-2 alloy produced by making use of a conventionalmelting technique and subjected to a final extrusion operation identicalwith the operation, to which the products Nos. 2, 3, 5 and 6 have beensubjected, and

FIG. 2 shows the variation in the flow stress as a function of thetemperatures for the same products.

ln both figures, the data regarding products Nos. 1 and 2 stated inTable l is marked as well.

What we claim are:

l. A method of producing dispersion-strengthened zirconium productscontaining fine metal oxide particles dispersed in a matrix of amaterial selected from the group consisting of zirconium and zirconiumalloys, comprising mixing the matrix material in a hydrided, powderycondition thoroughly with a powder of a metal oxide selected from thegroup consisting of yttrium oxide, magnesium oxide, cerium oxide andberyllium oxide and having a very small average particle size, andheating the mixture in vacuum for expelling hydrogen and for sinteringthe mixture, said heating being carried out at a temperature of below800C., and a temperature above 600 until the residual hydrogen in thesintered material is below about 20 ppm, and thereafter subjecting thesintered material to mechanical compacting treatment at an elevatedtemperature below 2. A method as claimed in claim 1, in which the secondof the two stages of the sintering operation, is carried out at atemperature in the region of 650-750:C.

3. A method as claimed in claim 1, in which the powder mixture is keptat a lower temperature ranging from 600C. up to approximately 700C, fora first period of time, andsubsequently, at a higher temperature for asecond period of time.

4. A method as claimed in claim 1, in which the second stage of thesintering operation is carried out at a pressure below approximately 10"mm Hg.

5. A method as claimed in claim 1, in which the metal oxide employed hasan average particle size which does not substantially exceed 0.5 u.

6. A method as claimed in claim 1, in which the metal oxide is added ina quantity that constitutes not more than percent by volume of the finalproduct.

7. A method as claimed in claim 1, in which the hydride employed has anaverage particle size which does not substantially exceed 10 .L.

8. A method as claimed in claim 1, in which the sintered product issubjected to a subsequent, mechanical compacting process by means ofextrusion while heated to a temperature of below 800C.

9. A method as claimed in claim 1, in which hydrided zirconium alloypowder is used as matrix powder.

10. A dispersion-strengthened zirconium product containing particles ofa metal oxide selected from the group consisting of yttrium oxide,magnesium oxide, cerium oxide and beryllium oxide, having an averageparticle size that does not substantially exceed 0.5 u, and ranging, byway of preference, between 0.01 and 0.5 .4., and being dispersed in asintered matrix of a material selected from the group consisting ofzirconium and zirconium alloys having an average particle size notexceeding 5 1., and to a predominant degree preferably lying in therange of l to 3 u.

11. A dispersion-strengthened zirconium product containing particles ofyttrium oxide having an average particle size that does notsubstantially exceed 0.5 u, and ranging, by way of preference, betwen0.01 and 0.5 u, and being dispersed in a sintered matrix of a materialselected from the group consisting of zirconium and zirconium alloyshaving an average particle size not exceeding 5 u, and to a predominantdegree preferably lying in the range of l to 3 1.1..

12. The method of claim 1 wherein the metal oxide is yttrium oxide.

13. The method of claim 1, wherein the mixture of metal oxide particlesand matrix material is subjected to a compression operation prior to thesteps of heating in vacuum and sintering.

14. The method of claim 12, wherein the mixture of metal oxide particlesand matrix material is subjected to a compression operation prior to thesteps of heating in vacuum and sintering.

15. The method of claim 1 wherein the sintering step is carried out intwo stages.

16. A method of claim 1, in which the metal oxide employed has anaverage particle size which lies in the range of from 0.01 ,u. to 0.5 M.

17. The method of claim 1, in which the hydride employed has an averageparticle size which lies in the range of from 0.5;1, to 3p 18. Themethod of claim 1 wherein heating the mixture in vacuum for expellinghydrogen and for sintering the mixture is performed at temperaturesbelow 750C.

2. A method as claimed in claim 1, in which the second of the two stagesof the sintering operation, is carried out at a temperature in theregion of 650*-750*C.
 3. A method as claimed in claim 1, in which thepowder mixture is kept at a lower temperature ranging from 600*C. up toapproximately 700*C., for a first period of time, and subsequently, at ahigher temperature for a second period of time.
 4. A method as claimedin claim 1, in which the second stage of the sintering operation iscarried out at a pressure below approximately 10 3 mm Hg.
 5. A method asclaimed in claim 1, in which the metal oxide employed has an averageparticle size which does not substantially exceed 0.5 Mu .
 6. A methodas claimed in claim 1, in which the metal oxide is added in a quantitythat constitutes not more than 15 percent by volume of the finalproduct.
 7. A method as claimed in claim 1, in which the hydrideemployed has an average particle size which does not substantiallyexceed 10 Mu .
 8. A method as claimed in claim 1, in which the sinteredproduct is subjected to a subsequent, mechanical compacting process bymeans of extrusion while heated to a temperature of below 800*C.
 9. Amethod as claimed in claim 1, in which hydrided zirconium alloy powderis used as matrix powder.
 10. A dispersion-strengthened zirconiumproduct containing particles of a metal oxide selected from the groupconsisting of yttrium oxide, magnesium oxide, cerium oxide and berylliumoxide, having an average particle size that does not substantiallyexceed 0.5 Mu , and ranging, by way of preference, between 0.01 and 0.5Mu , and being dispersed in a sintered matrix of a material selectedfrom the group consisting of zirconium and zirconium alloys having anaverage particle size not exceeding 5 Mu , and to a predominant degreepreferably lying in the range of 1 to 3 Mu .
 11. Adispersion-strengthened zirconium product containing particles ofyttrium oxide having an average particle size that does notsubstantially exceed 0.5 Mu , and ranging, by way of preference, betwen0.01 and 0.5 Mu , and being dispersed in a sintered matrix of a materialselected from the group consisting of zirconium and zircoNium alloyshaving an average particle size not exceeding 5 Mu , and to apredominant degree preferably lying in the range of 1 to 3 Mu .
 12. Themethod of claim 1 wherein the metal oxide is yttrium oxide.
 13. Themethod of claim 1, wherein the mixture of metal oxide particles andmatrix material is subjected to a compression operation prior to thesteps of heating in vacuum and sintering.
 14. The method of claim 12,wherein the mixture of metal oxide particles and matrix material issubjected to a compression operation prior to the steps of heating invacuum and sintering.
 15. The method of claim 1 wherein the sinteringstep is carried out in two stages.
 16. A method of claim 1, in which themetal oxide employed has an average particle size which lies in therange of from 0.01 Mu to 0.5 Mu .
 17. The method of claim 1, in whichthe hydride employed has an average particle size which lies in therange of from 0.5 Mu to 3 Mu .
 18. The method of claim 1 wherein heatingthe mixture in vacuum for expelling hydrogen and for sintering themixture is performed at temperatures below 750*C.